1. Field of the Invention
The invention relates to the separation of gases by the use of pressure using adsorption and membrane gas separation processes and systems. More particularly, it relates to the recovery of a high purity product gas and a high purity secondary product gas thereby.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes and systems are employed in a wide variety of industrial applications to produce high purity gas streams. In such processing, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component are commonly passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The bed is thereafter depressurized to a lower desorption pressure for desorption of the more readily adsorbable component from the adsorbent material and its removal from the bed prior to the introduction of additional quantities of the feed gas mixture to the bed as cyclic adsorption-desorption operations are continued in the bed. Such PSA processing is commonly carried out in multi-bed systems, with each bed undergoing the desired PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in each other bed in the system.
PSA systems are typically used in industrial applications to produce a single product stream from a given feed gas supply. For air separation purposes, PSA systems achieve the desired separation of oxygen and nitrogen because of the greater selectivity of the adsorbent employed for either oxygen or nitrogen. The adsorptive capacity of any particular adsorbent material increases at higher pressure levels and decreases at lower pressures. In PSA processes and systems for the production of high Purity oxygen product, the adsorbent employed may be one having a greater selectivity for either the desired oxygen product or for nitrogen. In systems in which the adsorbent employed is a nitrogen selective material, such as zeolitic molecular sieves, product oxygen is produced as the less readily adsorbable component removed from the bed during the adsorption step at the upper adsorption pressure. When oxygen is the desired product in systems employing an oxygen selective material, such as carbon molecular sieves, product oxygen is produced as the more readily adsorbable component upon the depressurization of the adsorbent bed to its lower desorption pressure. In PSA processes and systems in which nitrogen is the desired product, similar effects will pertain depending on whether the PSA system employs an oxygen or a nitrogen selective adsorbent.
Those skilled in the art will appreciate that PSA systems inherently can not completely separate any given feed stream component from the other components of the feed stream. In general, the PSA separation produces a product gas stream that contains a high percentage of one component together with a small amount of the remaining components. The other stream removed from the PSA system, i.e. the waste stream, will contain all of the incoming feed stream components. The fact that the adsorption system does not completely separate any component of the incoming feed stream from the other components is often the reason why a so-called waste stream exists in PSA processing. Quite frequently, this non-product waste stream does not contain a sufficiently high percentage of any given component to be of use in practical commercial operations. Therefore, this stream is of no significant value to the end user of the gas separation operation.
In the commercially important PSA-air separation technology, it is nevertheless desirable to recover the most prominent component of the waste stream, whether oxygen or nitrogen, as a separate high purity gas stream. Such recovery would serve to enhance the technical and economic feasibility of employing PSA operations in an ever-increasing field of industrially significant applications. In a typical adsorption process for air separation to produce oxygen, for example, the average oxygen purity of the waste stream will typically be about 10, with the remaining 90% of the waste stream comprising mostly nitrogen. Compared to the composition of air, the oxygen purity of the waste stream is reduced by more than 50% compared to that of air. Therefore, producing a desired purity of nitrogen from this waste stream would inherently require a smaller and less costly separation unit than is needed in the processing of a feed air stream, since more than half of the oxygen present in air has already been removed. Additionally, any compression equipment required in supplying this waste stream to a separation unit would be smaller and would consume less power that is required for air processing. Any approach to capturing the valuable component from the waste stream, however, must be capable of economically as having this desirable result, so that the cost of the additional operation does not exceed the savings obtained by so-capturing said component.
It is an object of the invention, therefore, to provide for the separation of a feed gas mixture using a PSA system and the recovery of a valuable component of the feed gas mixture from the PSA waste stream.
It is another object of the invention to provide a PSA air separation process and system capable of producing oxygen or nitrogen product, with enhanced means for recovering a high purity stream comprising the most prominent component of the waste stream thereupon.
With these and other objects in mind, the invention is herewith described in detail the novel feature thereof being particularly pointed out in the appended claims.